1. Technical Field
The present disclosure relates to a detection method in particular, to a frequency detection method used for a clock and data recovery (CDR) circuit operating without referencing an external reference clock signal.
2. Description of Related Art
In a general communication system, a transmitter operatively generates a data signal according to the clock signal at the transmitter and transmits the data signal to a receiver through a wireless or wired transmission medium. In order for the receiver to correctly identify the logic level (e.g., 0 or 1) of the data signal, the receiver has to read the data signal received after the clock signal at the receiver had synchronized with the clock signal at the transmitter. Conventionally, the receiver includes a clock and data recovery (CDR) circuit for recovering the clock signal at the receiver to be identical to the clock signal at the transmitter.
Generally, at least one clock generator is built-in the CDR circuit for generating a sampling clock signal having a specific frequency. The CDR circuit must ensure that the sampling signal can effective sample the data signal. Specifically, during the operation, the CDR circuit adjusts the frequency of an initial clock signal of the clock generator to the specific frequency before beginning to sample the data signal received. For example, when the frequency of an initial clock signal is too low, causing distortion in the sampling result, the CDR circuit generates a control signal to drive the clock generator to increase the frequency of the clock signal.
However, the clock generator, being built-in the CDR circuit, is known to be vulnerable to variations in semiconductor processing, temperature, and voltage generating the problem of frequency drifting. Accordingly, most of the conventional CDR circuits adjust an initial clock signal of the built-in clock generator according to a reliable and precise external reference clock signal. For example, the reference clock signal may be a clock signal transmitted to the receiver by the transmitter during data transmission or may be generated externally by a reliable off-chip crystal oscillator connected thereto.